Method and apparatus for deposition of multilayer device with superconductive film

ABSTRACT

A physical vapor deposition system includes a chamber, three target supports to targets, a movable shield positioned having an opening therethrough, a workpiece support to hold a workpiece in the chamber, a gas supply to deliver nitrogen gas and an inert gas to the chamber, a power source, and a controller. The controller is configured to move the shield to position the opening adjacent each target in turn, and at each target cause the power source to apply power sufficient to ignite a plasma in the chamber to cause deposition of a buffer layer, a device layer of a first material that is a metal nitride suitable for use as a superconductor at temperatures above 8° K on the buffer layer, and a capping layer, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application Ser. No.62/822,615, filed on Mar. 22, 2019, the disclosure of which isincorporated by reference.

BACKGROUND Technical Field

The disclosure concerns a reactor for processing a workpiece to deposita metal nitride, particularly a metal nitride that is suitable as asuperconductive material.

Background Discussion

In the context of superconductivity, the critical temperature (T_(C))refers to the temperature below which a material becomessuperconductive. Niobium nitride (NbN) is a material that can be usedfor superconducting applications, e.g., superconducting nanowire singlephoton detectors (SNSPD) for use in quantum information processing,defect analysis in CMOS, LIDAR, etc. The critical temperature of niobiumnitride depends on the crystalline structure and atomic ratio of thematerial. For example, referring to FIG. 1, cubic δ-phase NbN has someadvantages due to its relatively “high” critical temperature, e.g.,9.7-16.5° K.

Niobium nitride can be deposited on a workpiece by physical vapordeposition (PVD). For example, a sputtering operation can be performedusing a niobium target in the presence of nitrogen gas. The sputteringcan be performed by inducing a plasma in the reactor chamber thatcontains the target and the workpiece.

SUMMARY

In one aspect, a method of forming a structure including a metal nitridelayer on a workpiece includes pre-conditioning the chamber by flowingnitrogen gas and an inert gas at a first flow rate ratio into thechamber and igniting a plasma in the chamber before placing theworkpiece in a chamber that includes a metal target, evacuating thechamber after the preconditioning, placing the workpiece on a workpiecesupport in the chamber after the preconditioning, and performingphysical vapor deposition of a metal nitride layer on the workpiece inthe chamber by flowing nitrogen gas and the inert gas at a second flowrate ratio into the chamber and igniting a plasma in the chamber. Thesecond flow rate ratio is less than the first flow rate ratio.

In another aspect, a physical vapor deposition system includes chamberwalls forming a chamber, a support to hold a workpiece in the chamber, avacuum pump to evacuate the chamber, a gas source to deliver nitrogengas and an inert gas to the chamber, an electrode to support a metaltarget, a power source to apply power to the electrode, and acontroller. The controller is configured to cause the gas source to flownitrogen gas and the inert gas at a first flow rate ratio into thechamber and cause the power source to apply power sufficient to ignite aplasma in the chamber to pre-condition a chamber before a workpiece onwhich a metal nitride layer is to be deposited is placed in the chamber,and to cause the gas source to flow nitrogen gas and the inert gas at asecond flow rate ratio into the chamber and cause the power source toapply power sufficient to ignite a plasma in the chamber to deposit ametal nitride layer on the workpiece by physical vapor deposition afterthe workpiece is placed in the chamber. The second flow rate ratio isless than the first flow rate ratio.

In another aspect, a cluster tool for fabrication of a device having ametal nitride layer includes a load lock chamber to receive a cassetteholding workpieces, a central vacuum chamber, a plurality of depositionchambers arranged in a cluster formation around and coupled to thecentral vacuum chamber, a robot to carry a workpiece between the vacuumchamber and the load lock chamber and plurality of deposition chambers,and a controller. The plurality of deposition chambers include a firstdeposition chamber having a first target, a second deposition chamberhaving a second target, and a third deposition chamber having a thirdtarget. The controller is configured to cause the robot to carry thesubstrate to the first deposition chamber and to cause the firstdeposition chamber to deposit a buffer layer on the workpiece, to causethe robot to carry the substrate from the first deposition chamber tothe second deposition chamber and to cause the second deposition chamberto deposit a metal nitride layer suitable for use as a superconductor attemperatures above 8° K on the buffer layer, and to cause the robot tocarry the substrate from the second deposition chamber to the thirddeposition chamber and to cause the third deposition chamber to deposita capping layer on the metal nitride layer.

In another aspect, a physical vapor deposition system includes chamberwalls forming a chamber, a first target support to hold a first target,a second target support to hold a second target, and a third targetsupport to hold a third target, a movable shield positioned in thechamber and having an opening therethrough, an actuator to move theshield, a workpiece support to hold a workpiece in a lower portion ofthe chamber, a vacuum pump to evacuate the chamber, a gas source todeliver nitrogen gas and an inert gas to the chamber, a power source toselectively apply power to the first target, the second target or thethird target, and a controller. The controller is configured to causethe actuator to move the shield to position the opening adjacent thefirst target, cause the gas source to flow a first gas into the chamber,cause the power source to apply power sufficient to ignite a plasma inthe chamber to cause deposition of a buffer layer of a first material onthe workpiece on the workpiece support, cause the actuator to move theshield to position the opening adjacent the second target, cause the gassource to flow a second gas into the chamber, cause the power source toapply power sufficient to ignite a plasma in the chamber to causedeposition of a device layer of a second material that is a metalnitride suitable for use as a superconductor at temperatures above 8° Kon the buffer layer with the first material being different incomposition from the second material, cause the actuator to move theshield to position the opening adjacent the third target, cause the gassource to flow a third gas into the chamber, and cause the power sourceto apply power sufficient to ignite a plasma in the chamber to causedeposition of a capping layer of a third material on the device layerwith the third material being different in composition from the firstand second materials.

These aspects can include one or more of the following features.

The metal target may include niobium or a niobium alloy. The metalnitride layer may include niobium nitride or a niobium alloy nitride.The metal target may be substantially pure niobium, and the metalnitride layer may be substantially pure niobium nitride. The metalnitride layer may be δ-phase NbN. The plasma may sputter the metal ofthe metal target.

The second flow rate ratio may be 2-30% less than the first flow rateratio. The first flow rate ratio may be 4:100 to 1:1, and the secondflow rate ratio may be 3:100 to 48:52. The chamber may be evacuated to apressure lower than 10⁻⁸ Torr.

Pre-conditioning may include placing a shutter disk on the substratesupport. A robot may be configured to position a shutter disk into thechamber for the pre-condition of the chamber. Pre-conditioning mayinclude heating the shutter disk to a temperature, and performing thephysical vapor deposition may include heating the workpiece to the sametemperature. The temperature may be 200-500° C. Igniting plasma inpreconditioning and igniting plasma in deposition may use the same powerlevel.

A nitrogen ion concentration in the plasma may be measured with anoptical sensor. A flow rate of the nitrogen gas and/or the inert gas maybe adjusted in response to nitrogen ion concentration measured by thesensor to bring the nitrogen ion concentration to a desiredconcentration. The sensor is positioned outside the chamber, and whereinthe chamber walls include a window to provide optical access to thechamber for the sensor.

A sputter shield may be positioning in the chamber. The sputter shieldmay have an opening to provide a clear line of sight for the sensor tothe plasma.

A buffer layer may be formed on the workpiece before forming the metalnitride layer. The metal nitride layer may be deposited directly on thebuffer layer. The buffer layer may be a metal nitride of a metaldifferent than the metal of the target. The buffer layer may be aluminumnitride.

A capping layer may be formed on the metal nitride layer. The cappinglayer may include carbon, silicon, a metal different than the metal ofthe target, or a nitride of a material different than the metal of thetarget. The capping layer may be carbon, silicon nitride or titaniumnitride.

The first target may be a metal other than a metal of the second target.The first gas may include nitrogen gas. The second target may includeniobium. The second gas may include nitrogen gas. The third target mayinclude carbon, silicon, or a metal other than a metal of the secondtarget.

The shield may be rotatable and the actuator may be configured to rotatethe shield.

Some implementations may provide one or more of the followingadvantages. The process permits reliable or stable deposition of highquality NbN with a high critical temperature. This permits fabricationof devices, e.g., SNSPD, that operate at higher temperatures, thusmaking such devices more practical. Devices can be fabricated withhigher quantum efficiency and low dark current. Devices can also befabricated with reduced timing jitter and faster detection response. Abuffer layer, a superconductive film, and a capping layer can bedeposited by a single tool without removing the workpiece from vacuum.This can significantly improve process stability and manufacturability,and reduces risk of contamination, e.g., oxidation, which also helpspreserve the high critical temperature.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialfeatures, aspects, and advantages will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram illustrating phase of niobium nitride as a function ofprocessing temperature and atomic percentage nitrogen.

FIG. 2 is a schematic cross-sectional side view of a reactor to depositmetal nitrides.

FIG. 3 is a graph illustrating voltage on the target as a function ofnitrogen flow and critical temperatures measured for various nitrogenflow values.

FIG. 4 is a flow chart of a process for depositing a metal nitride.

FIG. 5 is a schematic cross-sectional view of a device that includes ametal nitride layer for use as a superconductive material duringoperation.

FIG. 6 is a schematic cross-sectional side view of a reactor to deposita seed layer, a metal nitride, and a capping layer.

FIG. 7 is a schematic top view of a cluster tool to deposit a seedlayer, a metal nitride, and a capping layer.

FIG. 8 is a schematic side view of a processing chamber to depositmultiple layers of different composition.

FIG. 9 is a schematic top view of the processing chamber of FIG. 8.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

As noted above, niobium nitride, particularly δ-phase NbN, has someadvantages as a superconductive material. However, δ-phase NbN can bedifficult to deposit at a satisfactory quality. For example, it may needa high vacuum (10⁻⁹ Torr or lower), as well as high mobility species(high temp, high peak power, and low duty cycle in pulsed DC). Althoughsemiconductor grade deposition tools can provide good uniformity, theytypically are configured for a lowest vacuum of about 10⁻⁸ Torr.However, increased pump capacity and additional traps for smaller atomicweight gases, e.g., water vapor, can improve vacuum performance.

Another issue is that a buffer layer, e.g., an aluminum nitride (AlN)layer, below the (super)conductive layer can help improve the criticaltemperature of the metal nitride layer. Similarly, a capping layer,e.g., a carbon layer or silicon nitride layer, above the(super)conductive layer can help protect the metal nitride layer, e.g.,to prevent oxidation. The capping and buffer layer would typically beprovided by separate deposition tools. Unfortunately, removing theworkpiece from the tool used for deposition of the metal nitride canresult in contamination or oxidation, thus reducing the criticaltemperature. However, a cluster tool can be configured to have multiplechambers, each of which can deposit a layer without removing theworkpiece from the vacuum environment, or a single chamber can beconfigured to deposit each of the layers, thereby avoiding the need toremove the workpiece.

Yet another issue is that even under good deposition conditions,reliably depositing a film with as high a critical temperature aspossible can be difficult. However, it has been discovered that thecritical temperature of niobium nitride as a function of nitrogencontent exhibits a hysteresis effect; different critical temperaturescan be obtained depending on whether the nitrogen content has beenramped up or ramped down. By performing a preconditioning of the chamberusing nitrogen gas before depositing the layer on the workpiece, theprocess can follow the more advantageous curve of this hysteresiseffect. As a result, a higher critical temperature can be obtained morereliably.

Deposition System

Referring now to FIG. 2, a physical vapor deposition (PVD) reactor 100includes a vacuum chamber 110. The chamber 110 is enclosed by chamberwalls, including a side wall 112, a floor 114 and a ceiling 116. Aworkpiece support 120, e.g., a pedestal or susceptor, can be positionedinside the chamber 110. The workpiece support 120 has a top surface 120a to support a workpiece 10 inside the chamber 110. The support 120 canbe elevated above the floor 114.

In some implementations, a temperature control system can control thetemperature of the support 120. For example, the temperature controlsystem can include a resistive heater embedded in or placed on thesurface 120 a of the workpiece support 120 and a power sourceelectrically coupled to the heater. Alternatively or in addition,coolant channels can be formed in the workpiece support 120, and coolantfrom a coolant supply can flow, e.g., be pumped, through the channels.

In some implementations, the vertical position of the workpiece support120 is adjustable, e.g., by a vertical actuator.

An opening 118 (e.g., a slit valve) can be formed in a wall of thechamber 110. An end effector (not shown) can extend through the opening118 to place the substrate 10 onto lift pins (not shown) for loweringthe substrate onto a support surface 120 a of the workpiece support 120.

In some implementations, the support 120, or a conductive electrode 121(see FIG. 8) within the support, is grounded. Alternatively, an externalpower source 136 (see FIG. 8), e.g., a DC or RF power source, can beused to apply a bias voltage or RF power to the support 120 or aconductive electrode 121 within the support 120, and thus apply a biasvoltage or RF power to the workpiece 10. Optionally, the power source136 can be coupled to the electrode 121 by an RF matching network 137(see FIG. 8).

A sputter shield 126 can be positioned inside the chamber 110 to preventsputtering of material onto the chamber side walls 112.

An electrode 130 forms a portion of the ceiling 116, and a target 140can be supported from the electrode 130. The electrode 130 iselectrically coupled to a power source 132. The power source 132 can beconfigured to apply a pulsed DC voltage. The power source 132 can becoupled to the electrode 10 by an RF matching network 133. The powerapplied can be 500 W to 20 kW, the voltage can be 200V to 600V, thefrequency can be 50 kHz to 250 kHz, and the duty cycle can be 60-100%.

The target 140 is a body, e.g., a disk, formed of the metal from whichthe metal nitride is to be deposited. On installation, the target can besubstantially pure metal, e.g., a body of substantially pure niobium.However, during processing nitrogen can react with the surface of thetarget, forming a surface layer of metal nitride.

A vacuum pump 150 is coupled to the chamber 110, e.g., by a passage withan opening in a region below the workpiece support 120, e.g., in thefloor 112, to evacuate the chamber 110. Examples of vacuum pumps includean exhaust pump with throttle valve or isolation valve, cryogenic pumpand turbo pump backed up by a mechanical pump. Although FIG. 2illustrates a single vacuum pump, in some implementations multiple pumpscan be used to increase the vacuum level. The assembly of one or morevacuum pumps 150 can bring the chamber down to a vacuum of less than8×10⁻⁹ Torr. The vacuum pumps 150 can maintain a rate of rise of lessthan 50 nTorr/min at the elevated deposition temperature.

A condensation plate 152 can be located in the passage connecting thechamber 110 to the vacuum pump 150. The condensation plate 152 providesa surface of or is positioned in the passage that is cooled sufficientlyfor water to condense on the plate 152. Thus, the condensation plate 152acts to capture/trap water vapor, and other lower-weight molecules thatcan condense, thus preventing such gas from forming part of the plasmaand reducing the impurity in the metal nitride film.

A gas source 160 is fluidically coupled to the chamber 110. The gassource 160 includes a source 162 of nitrogen gas (N₂) and a source 164of an inert gas, e.g., argon or helium. Flow rates of the nitrogen gasand the inert gas, and thus the ratio of flow rates, can be controlledby independently controllable valves 166 and mass flow controllers.Although FIG. 2 illustrates separate passages entering the chamber, thegasses could be mixed before entering the chamber 110, and morecomplicated gas distributor apparatus, e.g., a gas distribution plate orshowerhead, an array of radial passages through the side wall, etc.,could be used to distribute gas into the chamber 110.

Application of power at an appropriate frequency and power to theelectrode 130 by the power source 132 can ignite a plasma 111 in thechamber 110. In particular, a pulsed DC bias can be applied through theelectrode 130 to the sputtering target 140, and the workpiece support120 can be electrically floating. The resultant electric field in thechamber 110 ionizes the sputtering gas to form a sputtering plasma 111that sputters the target 140, causing deposition of material on theworkpiece 10. The plasma 111 is typically generated by applying a DCpower level between 100 Watts and 20 kWatts, e.g., 1-5 kWatts. The powersource 132 can supply DC pulses at a frequency 50 kHz to 250 KHz, e.g.,200 kHz. The duty cycle of the pulses can be 50-100%, e.g., 50-70% or60-100%.

In some implementations, a magnet assembly 170 is positioned outside thechamber 110, e.g., above the ceiling 116. The magnet assembly 170 canhelp confine the ions in the plasma and increase ion energy on thesubstrate 10.

A controller 190, e.g., a programmed general purpose computer having aprocessor, memory and non-transitory storage media to store a computerprogram, can be coupled to the various components to control theprocessing system 100.

An optical emission sensor 180 can be used to monitors the plasmaconcentration and/or to monitor the gas composition during deposition.The sensor 180 can be positioned outside the chamber 110, but be placedadjacent a window 182 through a wall of chamber, e.g., the side wall112, to have a view of the plasma 111. If necessary, an aperture 184 canbe formed in the shield 126 to provide the sensor 180 with a clear lineof sight to the plasma 111. The optical emission sensor 180 can measurethe nitrogen ion concentration in the plasma 111. In someimplementations, the optical emission sensor 180 can also measure theinert gas ion concentration in the plasma 111. The optical emissionsensor 180 can provide these measurements to the controller 190.

The controller 190 can be configured to control the gas source 160 toadjust the flow rate(s) of the nitrogen gas and/or the inert gas inresponse to the measured ion concentration(s). For example, thecontroller 190 can operate in a feedback loop to control the gas flowrate(s) to bring the nitrogen ion partial pressure to a desired partialpressure, or to bring the nitrogen ion concentration to a desiredconcentration. The controller 190 can also be configured to control gasflow rates of the gas source 160 to maintain a stable plasma and/orachieve a desired condition on the surface of the target 140.

Deposition of Niobium Nitride

The workpiece processing tool 100 can be employed to perform depositionof niobium nitride, in particular δ-phase NbN, on a workpiece. In oneexample, the workpiece 110 includes a buffered layer, e.g., aluminumnitride, onto which the niobium nitride is to be deposited.

FIG. 3 illustrates voltage on the target as a function of nitrogen flow.A fixed pulsed DC power was applied to the target. The potential of thetarget was measured with respect to the ground. This potential willchange depending on the sputtering yield of the target and ionconcentration in the plasma. In general, the target voltage can be astandin value for the critical temperature, albeit not as a linearrelationship.

In general, a higher quality film will exhibit both a higher criticaltemperature. As noted above, it has been discovered that the criticaltemperature of niobium nitride as a function of nitrogen contentexhibits a hysteresis effect. Still referring to FIG. 3, where thenitrogen flow rate is being increased for successive workpieces, thetarget voltage follows curve 202. It is believed that the surface of theniobium target goes from a metallic mode to a “poisoned” mode whensufficient N₂ is present in the chamber to form a thin layer of NbN onthe target surface. In contrast, where the nitrogen flow rate is beingdecreased for successive workpieces, the target voltage follows curve204. Again, it is believed that as the N₂ partial pressure decreases,the target starts to become “de-poisoned” and switch back to themetallic mode.

For both curves 202, 204, the target voltage has a maximum value justbefore a sudden drop. It is believed that this is because niobium-richniobium nitride (NbN) film is typically formed if the target is in themetallic mode, whereas a NbN film with good stoichiometry and desiredcubic phase is typically formed if the target is in the poisoned mode.

However, where the flow rate is being successively decreased (curve204), the drop-off of the target voltage occurs at a lower nitrogen flowrate. This may indicate that the partial pressure where the niobiumtarget is de-poisoned, is lower than the partial pressure at which theniobium target becomes poisoned.

In addition, where the flow rate is being successively decreased, thetarget voltage actually reaches a higher value (indicated at 206), ascompared to where the flow rate is being successively increased (curve202). Moreover, measurement of the critical temperatures of the niobiumnitride deposited at these flow rates confirms that a higher criticaltemperature can be achieved if the nitrogen flow rate has been decreasedrather than increased as compared to a prior process.

It is possible to take advantage of this hysteresis effect. Inparticular, by performing a preconditioning of the chamber usingnitrogen gas before depositing the layer on the workpiece, the processcan follow the more advantageous curve of this hysteresis effect. As aresult, a higher critical temperature can be obtained more reliably.

Referring to FIGS. 2 and 4, a process 260 for fabricating the metalnitride layer begins by preconditioning the chamber 110 (step 262). Thechamber 110 is evacuated. A shutter disk, e.g., a metal disk of aboutthe same diameter but thicker than a workpiece, can be placed on thesupport 120 (the workpiece on which the layer is to be deposited is notpresent in the chamber 110). The gas distribution assembly 160 suppliesnitrogen gas to the chamber 110.

The gas distribution assembly 160 can also supply an inert gas, e.g.,argon or helium, to the chamber 100. The inert gas can be used to dilutethe nitrogen gas; this can increase plasma density. The gas distributionassembly 160 can establish a total pressure (nitrogen and inert gas) of2 to 20 mTorr. In this preconditioning step, the nitrogen gas issupplied at a first flow rate, e.g., 15 to 40 sccm, e.g., 20 sccm. Thenitrogen gas and the inert gas can be supplied at a first ratio, 4:100to 1:4, e.g., 4:100 to 1:1, e.g., 2:1 to 1:1, of nitrogen to inert gas(the ratio can be a ratio of flow rates in sccm). Power is applied tothe electrode 130, e.g., as described above, to induce the plasma 111.

This conditioning process can be carried out with the shutter disk at atemperature of, e.g., 200-500° C. The preconditioning process canproceed, e.g., for 60-300 seconds.

After preconditioning, the chamber is evacuated again, e.g., drawn downto 10⁻⁹ Torr, the dummy substrate is removed, and the workpiece isplaced into the chamber 110 and onto the support 120.

Now the metal nitride can be formed on the workpiece by a physical vapordeposition process (step 264). The gas distribution assembly 160supplies nitrogen gas to the chamber 110, and power is applied to theelectrode 130, e.g., as described above, to induce a plasma 111. Thepower source 132 can apply the same RF power, frequency and duty cycleduring deposition as in preconditioning.

In the deposition step, the nitrogen gas is supplied at a second flowrate that is lower than the first flow rate, while the flow rate of theinert gas remains the same as in the preconditioning step. For example,the second flow rate of the nitrogen gas can be at least 2% lower, e.g.,at least 10% lower. For example, the second flow rate can be 2-30%lower, e.g., 10-30% lower. For example, the second flow rate can be15-18 sccm.

Alternatively, the flow rate of the nitrogen gas could be held constant,but the flow rate of the inert gas could be increased.

In the deposition step, the nitrogen gas and the inert gas can besupplied at a second ratio, e.g., 3:100 to 1:6, e.g., 3:100 to 45:52,e.g., 1.5:1 to 1:3, of nitrogen to inert gas (the ratio can be a ratioof flow rates in sccm). The second ratio can be less than the firstratio, e.g., at least 2% lower, e.g., at least 10% lower. For example,the second ratio can be 2-30% lower, e.g., 10-30% lower.

This physical vapor deposition process can be carried out with theworkpiece at a temperature of, e.g., 200-500° C. The workpiece can beprocessed at the same temperature as the shutter disk in thepreconditioning process. The deposition process can proceed, e.g., for10-600 seconds.

Application of power to the electrode 130 at appropriate frequency andduty cycle will ignite plasma in the chamber 110. The plasma will causesputtering of material from the target 130 onto the workpiece 10. Due tothe presence of nitrogen in the plasma, a combination of nitrogen andthe metal, e.g., niobium nitride, is deposited onto the workpiece. Thepreconditioning process can enable the deposition of NbN of the rightstoichiometry and crystal quality from the beginning to end of thedeposition process.

Appropriate processing conditions for the physical vapor deposition toform δ-phase NbN should be about in the ranges discussed above, althoughdifferences in process chamber configuration, etc., can causevariations. If necessary, appropriate process conditions can bedetermined empirically.

Finally, the chamber 110 is evacuated again and the workpiece isremoved.

Although the discussion above focuses on niobium nitride, thesetechniques can be applicable for other metal nitrides, e.g., a nitrideof a mixture of niobium with another metal, e.g., NbTiN.

Multilayer Device

FIG. 5 is a schematic illustration of some layers in a device 220 thatincludes a metal nitride layer 226 for use as a superconductivematerial. The device 220 could be superconducting nanowire single photondetectors (SNSPD), a superconducting quantum interference device(SQUID), a circuit in a quantum computer, etc. FIG. 6 is a flowchart ofa method of fabrication of the layers.

Initially, a buffer layer 224 can be deposited on a substrate 222 (step250). The substrate can be, for example, a silicon wafer. Althoughillustrated as a single block, the substrate 222 could include multipleunderlying layers.

The buffer layer 224 can be a material to help improve the criticaltemperature of the metal nitride, especially when the metal nitridelayer is thin. Alternatively or in addition, the buffer layer 224 canimprove adhesion between the metal nitride layer 226 and the substrate222. The buffer layer 224 can be dielectric or conductive but is notsuperconductive at the operating temperature of the device 200. In someimplementations, the buffer layer 224 is formed of a different metalnitride than the metal nitride used for layer 226. For example, thebuffer layer 224 can be formed of aluminum nitride (AlN), hafniumnitride (HfN), gallium nitride (GaN), or indium nitride (InN).Alternatively, the buffer layer 224 could be formed of a carbide, e.g.,silicon carbide. The buffer layer can have a (002) c-axis crystalorientation. The buffer layer 224 can be deposited by a standardchemical vapor deposition or physical vapor deposition process.

Next, the metal nitride layer 226 is deposited on the buffer layer (step260). The metal nitride layer 226 can be deposited using the two-stepprocess 260 and system 100 discussed above.

After the metal nitride layer 226 is deposited, a capping layer 228 canbe deposited on the metal nitride layer 226 (step 270). The cappinglayer 228 serves as a protective layer, e.g., to prevent oxidation ofthe metal nitride layer 226 or other types of contamination or damage.The capping layer 228 can be dielectric or conductive but is notsuperconductive at the operating temperature of the device 200. In someimplementations, the capping layer 228 is a nitride of a differentmaterial from the metal of the metal nitride used for layer 226. In someimplementations, the capping layer 228 is a metal different from themetal of the metal nitride used for layer 226. Examples of materials forthe capping layer 228 include carbon, silicon, titanium nitride (TiN),and silicon nitride (SiN). The buffer layer 224 can be deposited by astandard chemical vapor deposition or physical vapor deposition process.

Etching can be used to form trenches 230 through at least the metalnitride layer 226 to form the conductive lines or other structuresneeded for the device (step 280). Although FIG. 4 illustrates the trenchas extending through the buffer layer 224, metal nitride layer 226 andcapping layer 228, other configurations are possible. For example, ifthe buffer layer 224 and capping layer 228 are both dielectric, then theetching can extend through just the metal nitride layer 226. In thiscase, the etching step 280 could be performed before the step 270 ofdepositing the capping layer. As a result, the capping layer 228 coulddirectly contact the buffer layer 224 in regions between the metalnitride islands. For example, if the buffer layer 224 and capping layer228 are both dielectric, then the etching can extend through just themetal nitride layer 226. As another example, the etch can extend throughthe metal nitride layer 226 and the buffer layer 224 or the cappinglayer 228 (but not both).

Tool for Multilayer Fabrication

As noted above, removing the workpiece from a tool used for depositioncan result in contamination or oxidation, thus reducing the criticaltemperature.

One technique to avoid this issue is to use a cluster tool with multiplechambers, each of which can deposit a layer without removing theworkpiece from the vacuum environment. FIG. 7 is a schematic top view ofa cluster tool 300 to deposit the buffer layer, the metal nitride layer,and the capping layer. The cluster tool 300 includes one or more centralvacuum chambers 310, one or more fab interface units 315 to receivecassettes that hold workpieces, and one or more robots 320 to transferworkpieces from the fab interface units 315 to other processingchambers, between the processing chambers, and from the processingchambers back to the fab interface units 315.

The processing chamber of the cluster tool 300 includes one or morephysical vapor deposition chambers 325 for deposition of the bufferlayer, e.g., for deposition of aluminum nitride (AlN), one or morephysical vapor deposition chambers, e.g., a physical vapor depositionchamber 100 as described above, for deposition of the metal nitridelayer, and one or more physical vapor deposition chambers 330 fordeposition of the capping layer, e.g., for deposition of a carbon layer.Chambers can be separated by appropriate slit valves. The cluster tool300 can be controlled by a controller 350, e.g., a general purposeprogrammable computer.

Another technique to avoid the need to remove the workpiece from vacuumis to deposit each of the layers in a single chamber. FIG. 8 is aschematic side view of a physical vapor deposition reactor 400 todeposit multiple layers of different composition. For example, thephysical vapor deposition reactor 400 can be used to deposit the bufferlayer, the metal nitride layer, and the capping layer. FIG. 9 is a topview of the physical vapor deposition reactor 400 (FIG. 8 can beconsidered along section line 8-8 in FIG. 9).

The physical vapor deposition reactor 400 is constructed in a mannersimilar to physical vapor deposition reactor 100, but includes threeseparate targets 140 a, 140 b, 140 c (additional targets can be presentif needed for other layers). The targets can be supported on the ceiling116 of the chamber 110 of the reactor 400. Each target is supported on arespective electrode 130 a-130 c. The different electrodes 130 a-130 ccan be coupled to a common power source 132, or to different powersources 132.

A rotatable shield 410 is positioned inside the chamber 110 and isshared by all the electrodes 130. The shield 410 can be suspended fromthe ceiling 116 by shaft 420, and the shaft can be rotated (shown byarrow A) about a vertical axis 426 by an actuator 422. In someimplementations, the actuator 422 can also move the shield 410vertically (shown by arrow B).

The rotatable shield 410 can have a hole 412 to expose a correspondingtarget. The shield 410 advantageously limits or eliminatescross-contamination between the plurality of targets 140 a-140 c. Therotatable shield 410 can also have a pocket 414 for each target that isnot being sputtered. For example, in some embodiments where threeelectrodes 130 are provided, the shield 410 can include a hole 412 toexpose one target at a time, and two pockets 414 to house the targetsthat are not being sputtered. By rotating the shield 410, a differenttarget can be exposed and operated.

In some embodiments, the physical vapor deposition reactor 400 includesa plurality of grounding rings 430 to provide improved grounding of theshield 410 to the ceiling 116, e.g., when the shield 410 is in theretracted position.

The three targets 140 a, 140 b, 140 c are formed of different materials,e.g., the materials to be sputtered to form the buffer layer, metalnitride layer and capping layer, respectively. For example, a firsttarget 140 a can be composed of the non-nitrogen component, e.g., themetal, of the element or compound used for the buffer layer. Forexample, if the buffer layer is to be formed of aluminum nitride, thefirst target 140 a can be aluminum. The second target 140 b can be thenon-nitrogen component, e.g., the metal, of the compound used for thesuperconductive layer. For example, if the superconducting layer is tobe formed of niobium nitride, the second target 140 a can be niobium.The third target 140 c can be the non-nitrogen component of the elementor compound used in the capping layer, e.g., carbon, silicon ortitanium.

In operation, the actuator 422 rotates the shield 410 so that theaperture 412 is aligned with the first target 140 a and the othertargets are covered. The vacuum pump 150 evacuates the chamber 110, thegas source 160 supplies a sputtering gas to the chamber 110, and thepower source 132 applies a power to the electrode 130 a to generate aplasma in the chamber. The plasma can cause sputtering of the materialof the first target 140 a, resulting in physical vapor deposition of thebuffer layer onto the substrate 10. If appropriate, the gas source cansupply nitrogen or another gas that will form a compound with thematerial of the first target 140 a. For example, if aluminum nitride isto be deposited, the first target 140 a can be aluminum and the gassource can supply both an inert gas, e.g., argon, and nitrogen. If thematerial of the first target 140 a is to be deposited as a substantiallypure element, then the gas can include only inert elements, e.g., argonor xenon.

When deposition of the buffer layer is complete, the chamber 110 isevacuated, and the actuator 422 rotates the shield 410 so that theopening 412 is aligned with the second target 140 b and the other twotargets are covered. The material of the metal nitride layer, e.g.,niobium nitride, can be deposited according to the method describedabove.

When deposition of the metal nitride layer is complete, the chamber 110is evacuated, and the actuator 422 rotates the shield 410 so that theopening 412 is aligned with the third target 140 c. The material of thecapping layer can be deposited, in a manner similar to described abovefor the buffer layer. The chamber 110 can evacuated again, and theworkpiece removed, e.g., by a robot.

Controller

A controller, e.g., the controller 190 and/or controller 150, can beimplemented in digital electronic circuitry, or in computer software,firmware, or hardware, or in combinations thereof. The controller caninclude one or more processors, e.g., a controller can be a distributedsystem. One or more computer program products, i.e., one or morecomputer programs tangibly embodied in a machine-readable storage media,can be executed by, or control the operation of, the controller, e.g., aprogrammable processor, a computer, or multiple processors or computers.A computer program (also known as a program, software, softwareapplication, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a stand-alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile. A program can be stored in a portion of a file that holds otherprograms or data, in a single file dedicated to the program in question,or in multiple coordinated files (e.g., files that store one or moremodules, sub-programs, or portions of code). A computer program can bedeployed to be executed on one computer or on multiple computers at onesite or distributed across multiple sites and interconnected by acommunication network.

The operations of the controller described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The operations of the controller can also beperformed by, and apparatus can also be implemented as, special purposelogic circuitry, e.g., an FPGA (field programmable gate array) or anASIC (application-specific integrated circuit).

While particular implementations have been described, other and furtherimplementations may be devised without departing from the basic scope ofthis disclosure. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the drawingsillustrate only exemplary embodiments. The scope of the invention isdetermined by the claims that follow.

What is claimed is:
 1. A method of forming a device on a workpiece,comprising: carrying the workpiece with a robot from a load lock chamberthrough a central vacuum chamber into a first deposition chamber;depositing a buffer layer that is a nitride of a first metal on theworkpiece in the first deposition chamber; carrying the workpiece withthe robot from the first deposition chamber through the central vacuumchamber to a second deposition chamber; depositing a metal nitride layersuitable for use as a superconductor at temperatures above 8° K on thebuffer layer in the second deposition chamber, wherein the metal nitridelayer is a nitride of a second metal different than the first metal;carrying the workpiece with the robot from the second deposition chamberthrough the central vacuum chamber to third deposition chamber;depositing a capping layer on the metal nitride layer in the thirddeposition chamber.
 2. The method of claim 1, wherein the metal nitridelayer comprises niobium nitride or a niobium alloy nitride.
 3. Themethod of claim 1, wherein the capping layer comprises carbon, silicon,a metal different than a metal of a target used for depositing the metalnitride layer, or a nitride of a material different than the metal ofthe target.
 4. The method of claim 1, comprising evacuating the centralvacuum chamber and the second deposition chamber to a pressure less than10⁻⁸ Torr.
 5. The method of claim 1, wherein each of depositing thebuffer layer, depositing the metal nitride layer, and depositing thecapping layer comprises physical vapor deposition including generating aplasma to sputter a target.
 6. The method of claim 1, wherein depositingthe buffer layer, depositing the metal nitride layer, and depositing thecapping layer are performed without removing the workpiece from a vacuumenvironment.
 7. The method of claim 1, wherein the capping layercomprises a nitride of a third metal different than the first metal ofthe metal nitride layer and different than the second metal of thebuffer layer.
 8. A method of forming a device on a workpiece,comprising: carrying the workpiece with a robot from a load lock chamberthrough a central vacuum chamber into a first deposition chamber;depositing an aluminum nitride buffer layer on the workpiece in thefirst deposition chamber; carrying the workpiece with the robot from thefirst deposition chamber through the central vacuum chamber to a seconddeposition chamber; depositing a niobium nitride layer suitable for useas a superconductor at temperatures above 8° K on the buffer layer inthe second deposition chamber; carrying the workpiece with the robotfrom the second deposition chamber through the central vacuum chamber tothird deposition chamber; depositing a titanium nitride capping layer onthe niobium nitride layer in the third deposition chamber.
 9. A methodof forming a device on a workpiece, comprising: carrying the workpiecewith a robot from a load lock chamber through a central vacuum chamberinto a first deposition chamber; depositing a buffer layer comprising anitride of a first metal on the workpiece in the first depositionchamber, the nitride of the first metal having a (002) c-axis crystalorientation; carrying the workpiece with the robot from the firstdeposition chamber through the central vacuum chamber to a seconddeposition chamber; depositing a metal nitride layer comprising a secondmetal suitable for use as a superconductor at temperatures above 8° K onthe buffer layer in the second deposition chamber, wherein the secondmetal is different than the first metal; carrying the workpiece with therobot from the second deposition chamber through the central vacuumchamber to third deposition chamber; depositing a capping layer on themetal nitride layer in the third deposition chamber.
 10. The method ofclaim 9, wherein the capping layer comprises carbon, silicon, a metaldifferent than a metal of a target used for depositing the metal nitridelayer, or a nitride of a material different than the metal of thetarget.
 11. The method of claim 8, wherein each of depositing the bufferlayer, depositing the niobium nitride layer, and depositing the cappinglayer comprises physical vapor deposition including generating a plasmato sputter a target.
 12. The method of claim 8, wherein depositing thebuffer layer, depositing the niobium nitride layer, and depositing thecapping layer are performed without removing the workpiece from a vacuumenvironment.
 13. The method of claim 9, wherein the capping layercomprises a nitride of a third metal different than the first metal ofthe metal nitride layer and different than the second metal of thebuffer layer.